You have to setup the sip_nat.conf with the External IP and the Internal Network if you are behin a NAT Firewall.. You may be able to make calls if you do but people will not hear you on the other end. Your PBX will not know where all the traffic goes. So we edit /etc/asterisk/sip_nat.confwith the following.

externip=123.123.123.123
localnet=192.168.0.0/255.255.255.0

If you have more than one internal network you need to add it as well. Here is an example with 3 different networks so they have three entries for the localnet.

Here is what we did to make it work.

externip=123.123.123.123
localnet=192.168.0.0/255.255.255.0
localnet=172.16.0.0/255.255.0.0
localnet=192.21.10.0/255.255.255.0

Once we did this all calls worked on all three networks